Adsorption of silver on (001), (011) and (111) oriented tungsten single crystals

Work function changes caused by silver adsorption were measured on (001), (011) and (111) tungsten single crystal planes using electron beam method. The decrease of the work function was observed on all planes studied in the small coverage region. The result evidences that the increase previously observed for the average work function in this coverage region can not be connected with negative dipole formation with silver atoms.     

Field electron emission changes by face specific adsorption of aluminum oxide and corresponding decomposition products on tungsten {110} and {100} planes

After deposition of aluminum oxide on a tungsten field emission microscope (FEM) tip and stepwise heating, three stages of emission changes were observed on {100}. Stages I and II cause work function decreases of 0.28 and 0.07 eV, respectively. Stage III is characterized by a large increase (Δ??+3 eV). The changes are discussed in terms of interaction of decomposition products (oxygen and aluminum) and adsorption of aluminum oxide. On {110} only a single aluminum oxide layer growth, which results in a work function decrease to ?=4.69 eV, is observed. The field electron emission from this layer was measured between 1400°K and room temperature. The experimental values were compared with those determined from Christov's unified theory of field and thermionic emission. The {110} layer values coincide with those obtained earlier from an aluminum oxide covered tungsten {112}.     

Field electron emission from an aluminum oxide covered tungsten {112} plane at temperatures up to 1700°K

Field electron emission from an aluminum oxide mono-layer, face specifically adsorbed on a tungsten {112} plane, was measured between 1670°K and room temperature and at local field strength ranging from 17 MV/cm to 35 MV/cm. In a Fowler-Nordheim plot straight lines were obtained up to about 820°K. The work function at 650°K was determined to be 3.2 eV. The experimental values were compared with those determined by Christov's unified theory of field and thermionic emission. Deviations are attributed to the change in the shape of the potential barrier, which is caused by the aluminum oxide adsorption.     

A field emission study of silver on rhenium

Field emission measurements of the change in average work function ?f of rhenium with adsorbed silver indicate that a rhenium-silver dipole forms with silver positive, of moment μ₀=5.2±1.5 ×10^?30 C m and polarizability $\text{α=29±12}\text{A}\text{?}{}^3$. Measurement of the rate of thermal desorption yields a mean binding energy of 2.31 ± 0.04 eV for sub-monolayer silver and 2.69±0.04 eV for a 2.5 monolayer deposit. Changes in work function induced by adsorption of silver on low-index rhenium plane surfaces are characterised by the formation of well-defined states and in this, silver resembles gold. These states are thought to result from a relatively large difference between the binding energy of adatoms on the low-index planes and on the surrounding surfaces, and this differnce is maintained when the surfaces are covered with silver. At the lowest coverage, silver is believed to be absent from all four observed planes and the measured rise in work function is thought to be apparent and to result from a decrease in field strength on the plane due to extension of the plane area by surrounding adsorbed silver. The structures adopted by silver overlayers are not known, but it is argued that on (101?0) and (101?1?) the final state at high coverage has the Ag(111) surface structure. On (112?0) and (112?2?) the silver layer at high coverage is thought to have either Ag(110) or Ag(100) surface structure. The structures of intermediate states found on all four low-index planes remain unkown. Field emission spectroscopy shows that emission from clean (101?0) is free-electron like and confirms earlier observations that emission from (202?1) is not. Spectroscopy also reveals a feature in the spectrum from silver on (101?0) which may be identified with a known surface state on Ag(111), thus providing some support for the assignment of Ag(111) to the surface structure of thick silver layers (> 3 monolayers) on (101?0).     

Deformation enhanced photoemission from oriented aluminum single crystal surfaces

Plastic-deformation-enhanced UV photoemission from oriented single crystal surfaces of aluminum has been studied. Clean single crystals of aluminum oriented in [111] direction were flexurally fatigued under UHV conditions and for the first time photoelectron energy distribution curves were obtained as a function of fatigue life using the Hel (21.2 eV) and the HeII (40.8 eV) photon lines. The integrated photocurrent was measured as a function of fatigue life using the photon lines from helium and argon sources. A progressive decrease in work function was observed as the specimen was subjected to increasing number of fatigue cycles. At a strain amplitude of ±0.2%, a drop of 0.2 eV in the work function occurred after 3000 fatigue cycles. Further, significant changes in aluminum valence band emission as a result of fatigue have been observed along with the formation of slip-steps on the surface of the aluminum specimen as seen under the scanning electron microscope. This alteration in the surface microgeometry caused by fatigue is believed to be responsible for the observed changes in the work function and the electronic band structure. The change in the total integrated photoyield caused by fatigue is thus determined by work function and valence band changes.     

Stability and diffusion properties of self-interstitial atoms in tungsten:a first-principles investigation

Employing a first-principles method based on the density function theory,we systematically investigate the structures,stability and diffusion of self-interstitial atoms(SIAs) in tungsten(W).The <111> dumbbell is shown to be the most stable SIA defect configuration with the formation energy of ~9.43 eV.The on-site rotation modes can be described by a quite soft floating mechanism and a down-hill "drift" diffusion process from <110> dumbbell to <111> dumbbell and from <001> dumbbell to <110> dumbbell,respectively.Among different SIA configurations jumping to near neighboring site,the <111> dumbbell is more preferable to migrate directly to first-nearest-neighboring site with a much lower energy barrier of 0.004 eV.These results provide a useful reference for W as a candidate plasma facing material in fusion Tokamak.     

Adsorption of gold on low index planes of rhenium

On atomically rough areas of a thermally cleaned rhenium field emitter, adsorbed gold behaves like it does on tungsten. The average work function $\text{}\text{?}$gf increases at low average gold coverage $\text{}\text{?}$gq due to formation of gold-rhenium dipoles, and at high coverage a structural transformation in the gold layer leads to a $\text{}\text{?}$gq-independent work function. Broadly similar behaviour is found for gold on the low-index planes of tungsten, but on low-index rhenium planes gold behaves rather differently. When thermally cleaned at > 2200 K and annealed below 800 K, the work function, φ(clean), of (101&#x0304;1&#x0304;) takes one of two values 5.25 ± 0.04 eV, and 5.36 ± 0.04 eV, which are tentatively attributed to the two possible structures of this plane. Similar behaviour is expected and observed for (101&#x0304;0),but the values taken by φ(clean) are not well defined. Both forms of (101&#x0304;1&#x0304;) are thought to undergo reconstruction above 800 K forming a single structure with φ(clean) = 5.55 ± 0.03 eV. (112&#x0304;0) and (112&#x0304;2&#x0304;) each have only one possible structure, and in keeping with this, φ(clean) has a single well-defined value for each plane. The flatness of (101&#x0304;1&#x0304;) and (101&#x0304;0) leads to field reduction at their centres which produces an increase in their measured work functions by up to 10%. The initial increase in φ produced by gold condensed at 78 K and spread at low equilibration temperatures T_s on (112&#x0304;2&#x0304;), (101&#x0304;1&#x0304;) and (112&#x0304;0) is attributed to gold-rhenium dipoles, which, on the latter two planes approximate to the Topping model, giving dipoles characterised by μ₀(1011) = 0.1 × 10^?30 C-m with α = 10 ų and μ₀(112&#x0304;0) = 0.32 × 10^?30 C-m with α = 22 ų, where μ₀ is the zero-coverage dipole moment and α its polarizability. Failure of the Topping model on (112&#x0304;2&#x0304;) is attributed to its atomically rough structure. No dipole effect is seen on (101&#x0304;0). Energy spectroscopy of electrons field emitted at (202&#x0304;1&#x0304;) and (101&#x0304;1&#x0304;) demonstrates the non-free character of electrons in rhenium, while the small effect of adsorbed gold strengthens the belief that gold is bound through a greatly broadened 6s level centred 5.6 eV below the Fermi level and the dipolar nature of the bond supports this model. At higher values of T_s and $\text{}\text{?}$gq gold appears to form states which are well-characterised by a coverage-independent work function. (101&#x0304;0), (101&#x0304;1&#x0304;) and (112&#x0304;0) each form two such states, one in the range 2 < $\text{}\text{?}$gq < 4 (state 1), and the second at $\text{}\text{?}$gq > 4 (state 2). The atomic radii of gold and rhenium are thought to be sufficiently similar to allow the possibility that state 1 is a replication of the Re plane structure by gold. The high work function and thermal stability of state 2, taken together with the observed temperature dependence of the transformation of state 1 to state 2, encourages the belief that state 2 results from atomic rearrangement of state 1 into a close-packed Au(111) structure. State 2 also forms on (112&#x0304;2&#x0304;) and the absence of state 1 on this plane suggests some surface alloying at coverages below 4 $\text{}\text{?}$gq.     

Field emission and field ion microscope study of Ga,In and Sn on W: Structure,work function,diffusion and binding energy

Gallium, indium and tin were deposited on a tungsten tip by making a contact between the tip and these metals in the liquid state. The activation energies of diffusion of the adsorbates on tungsten were found to be 0.29 eV for Ga, 0.35 eV for In and 0.71 eV for Sn. The adsorbates were field-evaporated by gradually increasing a positive tip voltage by a small increment each time and the variation of the work function with the decreasing coverage was examined for each evaporation stage. The result indicates that the adatoms assume one of two different adsorption states. The adatoms bound as strongly as in a bulk crystal were field-evaporated at a low evaporation field. The remaining adatoms form a more strongly bound covering layer which maximizes the average work function of the covered surfaces, 4.75 eV for Ga, 4.63 eV for In and 5.10 eV for Sn, and are field-evaporated at a significantly higher field. The covering layer of the strongly bound adatoms were observed on the areas from the {001} to {114} planes and were hardly noticed on the {011} and 112 areas. The arrangement of the strongly bound adatoms, particularly on the {114} planes, is found to be a precise replica of the substrate arrangement. Thus, the surface density of the adatoms is exactly the surface density of the substrate atoms. The observed results suggest that an adatom occupying a tungsten lattice site and contacting four substrate tungsten atoms can establish unusually strong bonding with the substrate.     

Field emission studies of oxygen on silver tips

Clean [111] oriented silver field emitting tips have been exposed to oxygen at 10^?3 Torr for 1 min at temperatures ranging from ? 170 to 200°C. From 50 to 200°C, an adsorption structure is formed that is stable in oxygen. The structure is characterized by intensely emitting regions on either side of enlarged {110}, {210} and {310} faces and a dark region in the (111)-{100} zone line directions. For adsorption from ? 170 to 200°C, the structure of the patterns depends distinctly on the adsorption temperature because the coverages are different and adsorption is activated. Oxygen adsorption at 10^?3 Torr for 1 min at 0°C causes an increase in the average work function of 1.15 eV. At 0°C, silver was exposed increasingly at 10^?6 Torr until 6100 L was reached. The work function increased progressively by 0.61 eV for this exposure. The {111}, {100}, {311}, {211} and {533} faces are attacked first. Then, the {110} faces are attacked followed by the {210} {310} and {320}. Heating of the adsorption layer formed at 0°C produced no changes in pattern and work function up to 100°C. Between 100 and 200°C, a strong decrease in work function and changes in the pattern result from oxygen penetration into the bulk.     

The electronic work function of the different faces of tungsten

We present a semi-empirical theory of the electronic work function of the different faces of tungsten. All the parameters entering the theory, except one, are estimated independently. The one adjustable parameter relates to the isotropic contribution to the work function, and, can, in principle, be determined from a self-consistent calculation of the band-structure of the energy levels in the bulk of the metal. The calculated values for the work function are in reasonably good agreement with available experimental data for practically all of the crystallographic planes with the exception of the (100) plane. For the latter, the calculated value is 0.3 eV above the experimental value. It is suggested that a negative contribution to the surface dipole potential from surface states, that exist on this plane, may be the reason of this discrepancy.     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

Surface diffusion of potassium on theW (112) plane

Dependent on temperature and coverage, numerous spectral density functionsW(f) of the field-emission flicker noise of potassium adsorbed on the tungsten (112) plane were determined. The analysis in terms of the Timm and van der Ziel model gives surface diffusion energies between 0.55 and 0.83 eV for (average) coverages from 0.3 to 1.0 and diffusion coefficients between 2×10^–10 and 3×10^–9 cm²/s at 400 K. The results are compared with those obtained previously for the tungsten (111) region. Some conclusions as to the mechanism of diffusion and the manifestation of phase transitions between commensurate and incommensurate adlayer structures are discussed.On leave from Wrocaw University     

The surface ionization of gadolinium,terbium and dysprosium on the tungsten (110) surface

A study of the surface ionization of gadolinium, terbium, and dysprosium (Gd, Tb, and Dy) is described and the results compared with the predictions of the Saha-Langmuir equation. A tentative measurement of holmium is also included. The experimental index parameter, E, which should be equal to the difference between the first ionization potential of the element and the apparent work function of the surface, was found to exceed the predicted value by the following amounts: Gd, 0.66 ± 0.14 eV; Tb, 0.27 ± 0.13 eV; and Dy, 0.24 ± 0.12 eV. The absolute ionization efficiencies were estimated by comparison of our gadolinium results with those reported by Dresser and Hudson. The preexponential coefficients are found to exceed the theoretically predicted values by an order of magnitude. Surface ionization efficiency of these rare earths is larger than predicted by the Saha-Langmuir equation by factors ranging from 5 to 25 at 2500 K. The (110) face of a tungsten single crystal was used as the ionizing surface, and its work function was determined to be 5.46 ± 0.11 eV in an auxiliary study of the emission of tungsten ions.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

Adsorption and surface diffusion of titanium on tungsten in a field emission microscope

The adsorption, thermal desorption and surface diffusion of titanium on tungsten in ultra-high vacuum have been studied by field emission microscopy. The work function versus coverage curve has a minimum of 3.95 eV. The theory of metallic adsorbate-induced work function changes given by Gyftopoulos and Levine gives results which are in good agreement with our experimental values. In some experiments the work function minimum occurs at 3.65 eV. This value corresponds to the value of the work function of β-titanium. It is believed that α-titanium to β-titanium phase transformation occurs when the emitter tip is annelaed at 1100 °K to sperad the titanium uniformly over its surface. Surface diffusion of titanium on tungsten occurs with a sharp boundary at 800 °K and the activation energies for the (211)→(411) directions are 43.0 and $\text{42.3}\text{kcal}\text{mole}$ respectively. The activation energy of thermal desorption was dependent on the coverage and ranges from 115.3 to $\text{160.4}\text{kcal}\text{mole}$. A satisfactory qualitative correlation between the theory and experiment is established.     

The work function of perfect W(110) planes: Fowler-Nordheim studies

Fowler-Nordheim measurements have been carried out on tungsten specimens field evaporated at low temperature in an endeavor to resolve conflicting values of the work function for the W(110) plane. Interpretation of such measurements has in the past been difficult, because of unknown contributions from plane edges and patch fields. These contributions have been isolated by taking advantage of the unusually low reactivity of W(110) toward molecular hydrogen. Observations on perfect W(110) planes yield a work function of 5.47 eV.     

The work function of hydrothermally synthesized UO2 and the implications for semiconductor device fabrication

The photoelectric work function of nearly stoichiometric (111) and (100) hydrothermally grown UO₂ was measured to be 6.28 ± 0.36 eV and 5.80 ± 0.36 eV, respectively. Candidate metals for electrical contacts are identified for both rectifying and non‐rectifying contacts based on work function, lattice compatibility, and electrical conductivity.     

Electronic structure of Yb/H－Si (111) interface

Photoemission spectra are measured for Yb covered surface of wet-chemically-etched H－Si (111). The results reveal that the lattice structure of the H－Si (111) surface is stable against the deposition of Yb atoms. X-ray photoemission spectra indicate the formation of a polarized (dipole) surface layer, with the silicon negatively charged. Ultraviolet photoemission spectra exhibit the semiconducting property of the interface below one monolayer coverage. Work function variation during the formation of the Yb/H－Si (111) interface is measured by the secondary-electron cutoff in the ultraviolet photoemission spectral line. The largest decrease of work function is ～1.65eV. The contributions of the dipole surface layer and the band bending to the work function change are determined to be ～1.15eV and ～0.5eV, respectively. The work function of metal Yb is determined to be ～2.80±0.05eV.     

Field emission spectroscopy of gold on the (110) and (211) planes of tungsten

Adsorption of Au up to monolayer amounts on the (110) plane of a tungsten field emitter leads to a decrease in work function, and also to a decrease in effective emitting area. Energy distributions of field emitted electrons show no structure attributable to Au. These results can be explained by postulating that there is no tunneling from Au atoms or from beneath Au atoms. The most probable reason for this is that in (110) tunneling matrix elements from outward normal tungsten d-orbitals are stronger than the tunneling matrix element from the non-directional 6s Au orbital. Experiments on the (211) plane show behavior similar to that previously found on (111), i.e. a work function increase but strong emission from Au atoms.     

Reflectometric study of dangling-bond surface states and oxygen adsorption on the clean Si(111)7 × 7 surface

External differential reflection measurements were carried out on clean Si (111)7 × 7 surfaces in the photon-energy range of 1.0–3.0 eV at 300 and 80 K. The results at 300 K showed a main peak in the joint density-of-states curve for optical transitions from the filled dangling-bond surface-states band to empty bulk-conduction band levels at 2.9 ± 0.1 eV and a shoulder at the low-energy side. At 80 K the shoulder sharpened to a real second peak at 1.76 ± 0.04 eV. Oxygen adsorption in the submonolayer region appeared to take place preferentially on the surface atoms which were responsible for the second peak.